Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4, 2003 in the name of inventors Paul Meadows et al., which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. A spinal cord stimulation (SCS) system typically includes an implantable pulse generator and at least one electrode lead that carries electrodes that are arranged in a desired pattern and spacing to create an electrode array. Individual wires within the electrode lead(s) connect with each electrode in the array. The electrode lead(s) is typically implanted along the dura of the spinal cord, with the electrode lead(s) exiting the spinal column, where it can generally be coupled to one or more electrode lead extensions. The electrode lead extension(s), in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the implantable medical device is implanted. Alternatively, the electrode(s) lead may be directly coupled to the implantable pulse generator. For examples of other SCS systems and other stimulation systems, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties.
Of course, implantable pulse generators are active devices requiring energy for operation. Oftentimes, it is desirable to recharge an implanted pulse generator via an external charger, so that a surgical procedure to replace a power depleted implantable pulse generator can be avoided. To wirelessly convey energy between the external charger and the implanted pulse generator, the charger typically includes an alternating current (AC) charging coil that supplies energy to a similar charging coil located in or on the implantable pulse generator. The energy received by the charging coil located on the implantable pulse generator can then be used to directly power the electronic componentry contained within the pulse generator, or can be stored in a rechargeable battery within the pulse generator, which can then be used to power the electronic componentry on-demand.
FIGS. 1 and 2 illustrate one example of an external charger 10 capable of wirelessly transmitting energy to an implantable pulse generator (not shown) via inductive coupling. The external charger 10 includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, and an AC charging coil (not shown) mounted to the bottom of the PCB 16, and various electronic components 20, such as microprocessors, integrated circuits, capacitors, audio transducers, connectors, mounted to the top of the PCB 16. The external charger 10 further includes a power source, and in particular a battery 24, electrically coupled to the electronic components 20 via spring terminals 26 mounted to the PCB 16. The pulse generator 10 includes a case 30, which serves to house all of the afore-mentioned components in a suitable manner. The case 30 comprises a bottom half 32 and a top half (not shown) that mate with each other in a clam-shell arrangement to enclose the inner components. The external charger 10 may also include a power on/off button to allow a user to initiate a charging function, status indicators for providing visual and/or audible signals to the user, and recharging terminals (all not shown) to allow the battery 24 to be recharged.
As shown in FIG. 1, electrical current flowing through the AC charging coil induces a magnetic field in a direction perpendicular to the plane in which the charging coil 18 lies. Thus, when a face of the case 30 is oriented in close proximity to an implanted device, such that the AC charging coil 18 is parallel to a corresponding coil within the implanted device, the magnetic field generated by the charging coil 18 induces an electrical current within a corresponding coil to charge a battery within, or otherwise provide power, to the implanted device.
As can be appreciated, the size of the charger 10 is dictated, at least in part, by the power efficiency of the AC charging coil. Due to the close proximity between the electronic components 20 and associated circuit traces on the PCB 16 and the charging coil 28, the magnetic field generated by the charging coil 18 induces eddy currents on the surface of the PCB 18 and components 20. Eddy currents are undesirable because they transform magnetic energy into thermal energy, thereby reducing the power efficiency of the AC charging coil, as well as undesirably heating the electronic components 20. In addition, the eddy currents create noise within the signals generated within the electronic components 20.
There, thus, remains a need to provide a more power efficient external charger for an implantable medical device.